IRRITABILITY 225 



solutions— roots, rhizoids,* pollen-tubes,t etc.— are the ones 

 most sensitive to differences in the proportions of water in 

 their environment. Other organs are relatively or quite 

 indifferent. Most stems are not hydrotropically sensitive, 

 but the peculiar stem of the parasite, dodder {Cuscuta), 

 which must twine closely about living plants and extract 

 nourishment from them, is somewhat sensitive. J 



The size and number of root-hairs is inversely propor- 

 tional to the ease with which the plant obtains the water it 

 needs. The difficulty of absorbing water in sufficient quan- 

 tity to meet the demand made by transpiration and evapo- 

 ration ( see p. 141 ) from the aerial parts acts as the stim- 

 ulus to the young epidermal cells of the root to grow larger and 

 into such form that they will be able more readily to absorb 

 the water held on the surfaces of the soil particles. An 

 amount of water in the soil less than the optimum is fol- 

 lowed by elongation of the epidermal cells, and they grow 

 out as hairs. Seeds germinated in moist air instead of in 

 damp soil develop a larger number of still longer hairs. On 

 the other hand, the roots of seedlings growing in water are 

 nearly or quite bare of hairs. The stimulus to the produc- 

 tion of root-hairs may develop within the plant itself. It 

 may consist in the osmotic withdrawal of water from the 

 epidermal cells to make good the losses above. In any case, 

 water exerts the stimulus to growth, and the living proto- 

 plasm reacts to the stimulus. 



The root-hairs grow toward and around soil particles. 

 These hold films of water on the surface. The growth of the 

 root-hairs toward the soil particles is evidently positively 

 hydrotropic. The growth of the hairs around the particles 

 may be the combined result of the stimulus of contact with 

 the solid (see p. 241) and of water. 



The opposite result, the growth of parts away from water, 



* Molisch, H. Untersuchungen iiber den Hydrotropismus. Sitzb. d. K. K. 

 Akad. der Wiss., Wien, 188i. 



f Miyoehi, M. fiber Eeizbewegung der PoUenschlauche. Flora, Bd. 78, 

 1894. 



X Peirce, G. J. Contribution to the physiology of the Genus Cuscuta. 

 Annals of Botany, vol. VIII., 1894. 



